1. Field of the Invention
The present invention is directed in general to the field of information processing. In one aspect, the present invention relates to a system and method for transmitting channel rank feedback information from one or more receivers.
2. Description of the Related Art
Wireless communication systems transmit and receive signals within a designated electromagnetic frequency spectrum, but capacity of the electromagnetic frequency spectrum is limited. As the demand for wireless communication systems continues to expand, there are increasing challenges to improve spectrum usage efficiency. To improve the communication capacity of the systems while reducing the sensitivity of the systems to noise and interference and limiting the power of the transmissions, a number of wireless communication techniques have been proposed, such as Multiple Input Multiple Output (MIMO), which is a transmission method involving multiple transmit antennas and multiple receive antennas. Such wireless communication systems are increasingly used to distribute or “broadcast” audio and/or video signals (programs) to a number of recipients (“listeners” or “viewers”) that belong to a large group. An example of such a wireless system is the 3GPP LTE (Long Term Evolution) system depicted in FIG. 1, which schematically illustrates the architecture of an LTE wireless communication system 1. As depicted, the broadcast server 28 communicates through an EPC 26 (Evolved Packet Core) which is connected to one or more access gateways (AGW) 22, 24 that control transceiver devices, 2, 4, 6, 8, which communicate with the end user devices 10-15. In the LTE architecture, the transceiver devices 2, 4, 6, 8 may be implemented with base transceiver stations (referred to as enhanced Node-B or eNB devices) which in turn are coupled to Radio Network Controllers or access gateway (AGW) devices 22, 24 which make up the UMTS radio access network (collectively referred to as the UMTS Terrestrial Radio Access Network (UTRAN)). Each transceiver device 2, 4, 6, 8 device includes transmit and receive circuitry that is used to communicate directly with any mobile end user(s) 10-15 located in each transceiver device's respective cell region. Thus, transceiver device 2 includes a cell region 3 having one or more sectors in which one or more mobile end users 13, 14 are located. Similarly, transceiver device 4 includes a cell region 5 having one or more sectors in which one or more mobile end users 15 are located, transceiver device 6 includes a cell region 7 having one or more sectors in which one or more mobile end users 10, 11 are located, and transceiver device 8 includes a cell region 9 having one or more sectors in which one or more mobile end users 12 are located. With the LTE architecture, the eNBs 2, 4, 6, 8 are connected by an S1 interface to the EPC 26, where the S1 interface supports a many-to-many relation between AGWs 22, 24 and the eNBs 2, 4, 6, 8.
As will be appreciated, each transceiver device, e.g., eNB 2, in the wireless communication system 1 includes a transmit antenna array and communicates with receiver device, e.g., user equipment (UE) 15, having a receive antenna array, where each antenna array includes one or more antennas. The wireless communication system 1 may be any type of wireless communication system, including but not limited to a MIMO system, SDMA system, CDMA system, SC-FDMA system, OFDMA system, OFDM system, etc. Of course, the receiver/subscriber stations, e.g., UE 15, can also transmit signals which are received by the transmitter/base station, e.g., eNB 2. The signals communicated between transmitter 102 and receiver 104 can include voice, data, electronic mail, video, and other data, voice, and video signals.
Various transmission strategies require the transmitter to have some level of knowledge concerning the channel response between the transmitter and each receiver, and are often referred to as “closed-loop” systems. An example application of closed-loop systems which exploit channel-side information at the transmitter (“CSIT”) are precoding systems, such as space division multiple access (SDMA), which use closed-loop systems to improve spectrum usage efficiency by applying preceding at the transmitter to take into account the transmission channel characteristics, thereby improving data rates and link reliability. SDMA based methods have been adopted in several current emerging standards such as IEEE 802.16 and the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) platform. With such preceding systems, CSIT can be used with a variety of communication techniques to operate on the transmit signal before transmitting from the transmit antenna array. For example, preceding techniques can provide a multi-mode beamformer function to optimally match the input signal on one side to the channel on the other side. In situations where channel conditions can be provided to the transmitter, closed loop methods, such as MIMO preceding, can be used. Precoding techniques may be used to decouple the transmit signal into orthogonal spatial stream/beams, and additionally may be used to send more power along the beams where the channel is strong, but less or no power along the weak, thus enhancing system performance by improving data rates and link reliability. In addition to multi-stream transmission and power allocation techniques, adaptive modulation and coding (AMC) techniques can use CSIT to operate on the transmit signal before transmission on the transmit array.
With conventional closed-loop MIMO systems, full broadband channel knowledge at the transmitter may be obtained by using uplink sounding techniques (e.g., with Time Division Duplexing (TDD) systems). Alternatively, channel feedback techniques can be used with MIMO systems (e.g., with TDD or Frequency Division Duplexing (FDD) systems) to feed back channel information to the transmitter.
Current proposals for providing channel rank feedback in LTE systems use periodic feedback methods for the downlink MIMO channel whereby the feedback period is controlled by the base station and is signaled to the user. However, there are several problems associated with periodic feedback. For example, there are difficulties in regulating the feedback period creating overhead loss in channel rank feedback and/or loss in performance. Furthermore, delays in identifying changes in the rank of the downlink channel by the base station can lead to catastrophic (several in succession) errors, thereby severely degrading performance.
Another problem related to channel rank information in MIMO systems is the uncertainty regarding whether a channel rank request sent by a UE is actually received by the Node B. In a MIMO system, when the UE feeds back the MIMO channel rank to the Node B, the Node B can always override the rank request and transmit to the UE at a lower rank. In this case, the UE does not know whether there was an error in channel rank feedback, in which case it can resend the rank feedback, or whether the transmitter decided to override the rank request. In some MIMO systems, this lack of information may cause the UE to unnecessarily resend the rank feedback thereby increasing uplink overhead.
Accordingly, an efficient feedback methodology is needed to provide the channel rank information to the transmitter while sustaining a minimal loss in link performance. In addition, there is a need for a methodology to provide an indication to the UE of an override of a rank request. Further limitations and disadvantages of conventional processes and technologies will become apparent to one of skill in the art after reviewing the remainder of the present application with reference to the drawings and detailed description which follow.
It will be appreciated that for simplicity and clarity of illustration, elements illustrated in the drawings have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to other elements for purposes of promoting and improving clarity and understanding. Further, where considered appropriate, reference numerals have been repeated among the drawings to represent corresponding or analogous elements.